Centrifugal clutches in series with brake

ABSTRACT

A safety device for disengaging the driving portion of a centrifugal clutch of a chain saw when a safety grip is released. A drive disc on the motor shaft has a pivotal centrifugal drive arm for driving the centrifugal clutch hub yoke which is rotatable on the motor shaft. The drive arm is held in operative position against the centrifugal force by a locking arm, which is withdrawn when the safety grip is released, thereby releasing the drive arm and disengaging the drive to the centrifugal clutch. Simultaneously, a brake stops the clutch drum and thus the chain.

United States Patent Johansson Jan. 15, 1974 [54] CENTRIFUGAL CLUTCHESIN SERIES 2,974,769 3/1961 Henderson 1921103 c WITH BRAKE 3,131,7975/1964 Bochan 192/114 R X Rolf Anders Gunnar Johansson, V are, 8-310,Unnaryd, Sweden Filed: Feb. 22, 1972 Appl. No.1 228,061

Inventor:

US. Cl 192/48.5, 30/383, 192/104 C, 192/145, 192/28, 192/17 C, 192/114,

Int. Cl. F16d 21/08, Fl6d 67/02 Field of Search 192/48.5, 48.3, 48.2,192/48.4, 28, 17 C, 114, 145, 48.7

References Cited UNITED STATES PATENTS 3/1937 Junkers 192/114 R X 8/1948Gravina et a1. l92/48.3 X

Primary ExaminerBenjamin W. Wyche Attomgy- Pierce, Sheffler & Parker[57] ABSTRACT A safety device for disengaging the driving portion of acentrifugal clutch of a chain saw when a safety grip is released. Adrive disc on the motor shaft has a pivotal centrifugal drive arm fordriving the centrifugal clutch hub yoke which is rotatable on the motorshaft. The drive arm is held in operative position against thecentrifugal force by a locking arm, which is withdrawn when the safetygrip is released, thereby releasing the drive arm and disengaging thedrive to the centrifugal clutch. Simultaneously, a brake stops theclutch drum and thus the chain.

11 Claims, 10 Drawing Figures CENTRIFUGAL CLUTCHES IN SERIES WITH BRAKEThe present invention relates to a safety device for centrifugal clutchmechanisms operative between a motor and a device powered thereby, e.g.,a tool, and in particular to a safety device for motor driven chainsaws.

For safety reasons, it is often desirable that motor driven toolsprovided with centrifugal clutch mechanisms are equipped with meanswhereby the tool, e.g., the saw chain of a motor driven saw, can bestopped rapidly. It is desirable, in the case of certain particularlydangerous tools that the tool can be stopped practically immediatelyupon actuating. a safety device. For a motor driven chain saw, forexample, the saw chain should be arrested within one-tenth second. Themain object of the present invention is therefore to provide means forthis purpose and the invention is based on the concept that the powertool can be stopped practically instantaneously when braking the same bydisconnecting the tool from the kinetic mass of the drive motor.

This object is achieved by means of the present invention which ismainly characterized in that the hub yoke of the centrifugal clutch isrotatably mounted on the motor dirven drive shaft and is driven therebyby means ofa coupling means capable of being disengaged by a safetydevice tripping means.

The invention will now be described in detail with reference to theaccompanying drawings which illus- ,trate a number of embodiments of theinvention applied to a motor driven saw. In the drawings,

FIG. 1 is a diagrammatic over-all view of a motor driven saw providedwith the safety device of the present invention,

FIG. 2 is a side view in larger scale of the stop ring and stop arm ofthe centrifugal clutch,

FIG. 3 is a general view of the centrifugal clutch and safety deviceseen in section through the line IIIIII in FIG. 2,

FIG. 4 is a sectional view of the stop arm taken through the line IV-IVin FIG. 2,

FIG. 5 illustrates the drive pulley of the safety device seen from thecentrifugal clutch,

FIG. 6 illustrates the drive pulley of FIG. 5 in section through theline VI-VI,

FIG. 7 is a side view of a synchronizing means for use with the deviceof the present invention,

FIG. 8 is a side view of the hub yokes of the centrifugal clutchprovided with clutch shoes,

FIG. .9 is a sectional view taken through the line IXIX in FIG. 8, and

FIG. 10 illustrates an alternative braking and triggering means in sideview and partly cut away.

The motor driven saw diagrammatically illustrated in FIG. 1 comprises ahousing 1, a rear handle 3, a front handle 5 and a guide bar 7. A chaindrive 9 cooperating with a chain bracket (not shown) passing around theguide bar 7 is driven in a manner hereinafter described by the motorbrackets (not shown) of the motor driven saw by means of a centrifugalclutch and a safety coupling constructed in accordance with theinvention. The safety coupling of the invention disengages thecentrifugal clutch from the motor shaft when a triggering device isactuated. In the embodiment illustrated in FIGS. 1-9, the triggeringdevice comprises a stop or release arm 11 engaging the safety coupling,FIGS. 1-4 the arm 11 being pivotally attached to the housing I of thesaw by means ofa pin 13. The arm 11 is arranged in a manner to permit itto be swung down into engagement with the safety coupling against theaction of a spring 15 by means of the armature 17 of an electromagnet 19mounted in the housing 1 of the saw. The electro-magnet 19 is energizedfrom a generator coil 21, one side of which is grounded to the sawhousing 1 via a line 23, while the other end is connected with one sideof the coil of the electro-magnet via a line 25. The supply of currentthrough the electro-magnet, and thus actuation of the safety coupling,is controlled in the ground connection of the electro-magnet.

As will be seen from FIG. 1, the electro-magnet 19 is connected with acontact 29 located in the rear handle 3 of the motor saw through a line27. A safety hand grip 31 in the rear handle 3 is capable of pivotingaround a shaft 33 and is connected with the body of the apparatus sothat the eIectro-magnet I9 is connected with said body when the handgrip 31 is released into aubtment with the contact 29. A safety handgrip corresponding to the hand grip 31 may also be provided in the fronthandle 5 of the saw. The embodiment illustrated in FIG. I, however, isprovided with another safety operating means in the front handle. Astirrup member 35 is mounted for rotation in front of the front handlearound a shaft 37. The stirrup member 35 is connected with the body ofthe apparatus and adapted to co-act with a contact 39 when pivotedforwardly, the contact 39 being connected through a line 41 with theline 27 extending to the electro-magnet 19. Thus, if the hand with whichthe front handle 5 is grasped during a working operation should sliptherefrom, the hand will strike the stirrup 35 and swing it forwardly,whereupon the safety device becomes operative and the stop arm 11 willdisengage the safety coupling.

The motor saw illustrated in FIG. 1 is provided with a further safetyoperating means in the form of an acceleration sensitive transmitter 43arranged in the guide bar 7 and which, via a line 45, is connected withthe line 41. The transmitter 43 is arranged to ground the electro-magnet19 to the saw housing when the guide bar 7 is subjected to violentupward movement (kick-back), thus disengaging the safety coupling.

The centrifugal coupling and safety coupling will now be described inmore detail with reference to FIG. 3.

The chain drive 9 is mounted for rotation on needle bearings 47 on thedrive shaft 49 of the saw and are maintained in position axially on theshaft by means of a plate 51 and a nut 53 secrewed onto a threaded endof the drive shaft 49. The chain drive 9 is formed integrally with theclutch drum 55 of the centrifugal clutch. The centrifugal clutch alsoincludes a hub yoke 59 (see also FIGS. 8 and 9) mounted on the driveshaft 49 by means of a needle bearing 57. The hub yoke is in the form ofa three-armed star on which clutch shoes 61 are arranged for radialmovement but are caused to rotate in a known manner. The clutch shoes 61are provided with a frictional coating 63 attached peripherally as byglueing and adapted to co-act with a flange 65 of the clutch drum 55projecting over the clutch shoes so that the drum is driven in a knownmanner by the hub yoke 59 of the centrifugal clutch when the rotationalspeed of the saw exceeds a certain engaging speed, for example 3,500rpm.

In accordance with the invention, however, the hub yoke 59 is notfixedly attached to the drive shaft 49 but is connected thereto by meansof a safety coupling 67, see FIGS. 3, and 6.

The safety coupling comprises a drive disc 69 fixedly connected to thedrive shaft 49. The drive disc 69 may be connected to the shaft by forcefitting the disc on a non-round portion 71 located on said shaft 49.

When comparing FIGS. 6 and 3 it should be observed that the sectionillustrated in FIG. 6 is taken along the line VIVI in FIG. 5 while thedrive disc illustrated in FIG. 3 is cut along the line III-III in FIG.5.

The drive disc 69 comprises a generally flat disc having connected tothe periphery thereof two pivot pins 73, 75. Pivotally attached to eachpin 73 is a centrifugal drive arm 77, on the side of which facing thedrive shaft 49 is arranged a shoulder 79. The shoulder 79 is arranged toco-act with a shoulder 81 located on a cam 83 formed integrally with thehub yoke 59 of the centrifugal clutch and located on the side of the hubyoke 59 remote from the clutch drum 55. The shoulder 79 is held inengagement with the shoulder 81 by means of a tension spring 85extending between the free end of the drive arm 77 and a pin 87 mountedon the drive disc substantially diametrically opposite the pivot pin 73.

When the motor reaches a certain rotational speed, for example 3,000r.p.m., the centrifugal force on the arm 77 will be greater than theforce of the spring 85, and hence the arm 77 will move outwardly in amanner to cause the shoulder 79 to disengage from the shoulder 81. Thismovement of the arm 77 is prevented, however, by a lock 89 which ispivotable around the second pivot pin 75 on the drive disc 69. In theposition illustrated in FIGS. 5 and 6, the rounded inner end 91 of thelock 89 engages a recess 93 located in the drive arm 77 and therebyprevents the arm 77 from moving outwardly under the influence ofcentrifugal force. The outer end 95 of the lock 89 projects beyond theperiphery of the drive disc 69 and is made heavier than the innerportion 91 of the lock 89, whereby the lock 89 takes the positionillustrated in FIG. 5 when acted upon by centrifugal force. If, as aresult of external influence by the outer end 95, the lock 89 is pivotedcounterclockwise when the drive disc 69 rotates, the inner end 91 of thelock 89 will leave the recess 93 in the arm 77 which is thus released tomove outwardly under the action of centrifugal force at speeds exceeding3,000 r.p.m. in a manner to cause the shoulder 79 to be disengaged fromthe shoulder 81. Under the influence of centrifugal force, thecentrifugal drive arm 77 will hold the lock 89 in a position generallyperpendicular to the position illustrated in FIGS. 5 and 6.

For the purpose of balancing the masses of the centrifugal drive arm 77and the lock 89, the drive disc is provided with a counterweight 97formed integrally with the disc and located diametrically opposite thearm 77 and the lock 89.

The lock 89 is operated by means of the aforementioned release arm 11.For this purpose, the release arm 11 is provided with a shoulder 99 onthe free end thereof. The shoulder 99 strikes the outer end 95 of thelock 89 in a manner to cause the lock to be rotated anti-clockwise whenthe arm 11 is actuated by the electro-magnet 19.

The release arm 11 is also arranged to initiate braking of the clutchdrum 55.

For this purpose, a brake ring 101 is arranged around the flange 65 ofthe clutch drum 55. The inner surface of the ring 101 and the outersurface of the flange 65 are provided with opposing, peripherallyextending grooves 103 and 105, respectively. Located between the grooves103 and 105 are two cylindrical springs 107 and 109. The springs aretensioned between attachments (not shown) located on the ring 101. Theperipheral surface of the ring 101 is in the form of a camming surfacehaving two shoulders 111, 113 arranged to co-act with the arm 11 whensaid arm is actuated by the electro-magnet 19. When the shoulder 111 orthe shoulder 113 engages the end of the arm 11, rotational movement ofthe ring 101 ceases, whereby the springs 107 and 109 cause a frictionalforce to be applied to the clutch drum 55 to stop the same. To reducethe force of impact between the shoulders 111 and 113 and the end of thearm 11, a shock-absorbing means is arranged on the arm 11. To this end,the inner end of the arm 11 is provided with a groove 115 which permitsthe arm 11 to move in its longitudinal direction relative to the pin 13.Arranged in the groove 115 is a spring 117 which normally holds the arm11 extended and which absorbs the impact energy.

The safety coupling 67 is also provided with means for preventing hardimpacts between the shoulder 79 on the arm 77 and the shoulder 81 on thecam 83. The said means is in the form ofa synchronizing device andcomprises two curved leaf springs 119, 121 (FIGS. 7 and 6) which arefastened together by rivets 123, 125 in a manner to present outwardlyprojecting free ends. The synchronizing springs are mounted with therivets 123, 125 engaging in grooves 127, 129 in the hub of the drivedisc 69 (FIG. 6). When the drive disc 69 rotates, the springs 119, 121are forced to accompany the movement thereof, wherewith the ends of thesprings drag against the radial inner wall of a recess 131 located inthe cam 83 of the hub yoke 59 of the centrifugal clutch (FIGS. 9 and 3).This means that the hub yoke 59 does not cease to rotate when the motoris disengaged but can be loosely carried by the disc 69 as it rotates sothat subsequent engagement of the arm 77 and the cam 83 can be effectedwithout excessive differencies in rotational speeds.

The mode of operation of the centrifugal clutch on the safety couplingwill now be described in detail, it being assumed by way of example thatthe engaging speed of the centrifugal clutch is 3,500 r.p.m. and thatthe centrifugal force acting on the centrifugal arm 77 is greater thanthe force of the spring 85 at a speed of 3,000 r.p.m. or higher.

When the motor saw is to be started, the saw is held by the front handleand the starting cord pulled. The safety handle 31, the so-calleddead-mans-grip located in the rear handle 3 of the motor saw, is thusnot depressed, and hence the eIectro-magnet 19 is energized and forcesthe arm 11 down into its position of engagement. As soon as the motorstarts, the lock 89 of the safety coupling 67 will engage the shoulder99 of the arm 11, thereby releasing the drive arm 77. At low speeds, thecentrifugal coupling is disengaged so that the chain drive 9 and thechain are maintained stationary. If the throttle of the motor, i.e., thefinger grip 133 shown in FIG. 9, is latched in a position to permit apart flow of fuel to pass to the motor, the motor during the start willspeed up to a rotational speed in excess of 3,000 r.p.m., whereby thecentrifugal drive arm 77 is thrown outwardly to cause the hub yoke 59 tobe disengaged from the drive disc 69. Thus, during a starting sequence,the motor is disengaged and the chain at rest.

When the motor saw is to be used in a sawing capacity, the safety handle31 is gripped, whereby flow of 5 current to the electro-magnet ceases.When this occurs, the arm 11 is drawn by the spring 15 away from theshoulder 111 or 113 on the ring 101, and the shoulder 99 simultaneouslyreleases the lock 89. It the motor has a rotational speed exceeding3,000 r.p.m., the drive arm 77 will take its outer position and does notdrive the hub yoke 59. The arm 77 also holds the lock 89 in a tangentialposition. The motor is thus still disengaged, despite the fact that thesafety handle 31 has been pressed in. In order for the motor to beengaged, its rotational speed must be decreased. When the rotationalspeed of the motor has been lowered to 3,000 r.p.m. or less and thesafety handle 31 is still held pressed in, the spring 85 will draw thearm 77 inwardly in a manner to cause the shoulder 79 to engage theshoulder 81 and drive the hub yoke 59. The lock 89 is then swung to aradial position under the influence of centrifugal force, therebyreaching the condition illustrated in FIG. 5. Since the engagement takesplaces at a rotational speed below 3,500 r.p.m., the centrifugal clutchis disengaged and the chain does not therefore begin to move. Thecentrifugal clutch is not engaged to drive the chain until therotational speed is re-increased.

If the rotational speed is less than 3,000 r.p.m. when the safety handleis gripped subsequent to a starting sequence, only the lock 89 isreleased, the lock thereby locking the drive arm 77 in the engagementposition occupied thereby. The centrifugal coupling has disengaged thechain.

When the safety handle 31 is released or the stirrup 35 actuated duringa working operation, the electromagnet 19 forces the arm 11 into itsposition of engagement, wherewith the ring 101 becomes operative andbrakes the clutch drum and the chain. The arm 11 also actuates the lock89. If the rotational speed of the motor exceeds 3,000 r.p.m., the arm77 will then be disengaged from the shoulder 81 on the hub yoke 59 todisengage the motor. In this way, the clutch drum 55 and the chain arebraked rapidly to a stand-still. If the rotational speed of the motor isless than 3,000 r.p.m., engagement between the centrifugal arm 77 andthe shoulder 81 is not broken, but since this rotational speed is belowthe coupling speed 3,500 r.p.m. of the centrifugal clutch, the motor isalready disengaged and the chain stopped. Since the motor is alwaysdisengaged in connection with braking movement of the chain, the kineticenergy of the motor need not be braked and neither is the motor stoppedwhen the safety device is brought into operation.

Although the invention has been described with respect to a certainembodiment thereof, it is not restricted to such embodiment, numerousmodifications being possible within the scope of this invention. Thus,the drive between the drive disc 69 and the hub yoke 59 need not beeffected by means of only one drive arm but a plurality of such arms maybe provided. Neither need the drive arm or arms be mounted for movementaround a pin on the drive disc but may be pivotable in other suitablemanners. Further, other engagement means than pivotable centrifugal armsare conceivable, for example radially'displaceable slides. In theillustrated embodiments, engagement between the drive disc and hub yokeonly takes place in one direction of rotation, although the drive armand hub yoke may alternatively be provided with engagement means whichact in both directions of rotation.

Such an arrangement may be of particular advantage on idling withtwo-stroke motors, in which the angular speed of the motor is veryirregular and heavy impacts would otherwise repeatedly occur in thedriving mechanism. Further, braking need not be effected on the drum ofthe clutch but can be effected at any suitable place on the powertransmission means between the drum and the driven tool.

The lock 89 need not be actuated by a locking arm as illustrated but canbe actuated by a movable cam. The cam can be combined to advantage witha strap brake for the clutch drum instead of the illustrated arrangementof the spring brake with the brake ring, thereby providing a simpleconstruction which experiences smaller mechanical stresses. An exampleof such a construction is illustrated in FIG. 10, which shows thecombined centrifugal clutch and the safety coupling as viewed from thedrive disc, which is partially cut away. With the illustratedconstruction of FIG. 10, the electrical actuation of the safety couplinghas been replaced with a mechanical actuating means. Alternatively, ahydraulic or pneumatic actuating means can also be used.

For the sake of simplicity, like parts in the construction of FIG. 10and the embodiment of FIGS. l-9 have been identified with like referencenumerals. The drive disc 69 with the centrifugal drive arm 77 and lock89, the hub yoke 59 with the cam 83 and co-rotating clutch shoes 61, andthe clutch drum 55 with flange 65 are generally similar in constructionto those previously described. One exception, however, is that theflange 65 of the clutch drum 55 is not provided with an outer groove andthat no brake ring 101 or springs 107, 109 are provided in theconstruction of FIG. 10. This construction is also a mirror-image of theconstruction of FIGS. 1-9, and the shaft 49 does not extend through thedrive disc 69.

The safety coupling is operated and braking of the clutch drum effectedin the following manner: A circle arcuate braking band is connected atone end to the housing 1 of the motor saw. The brake band 135 extendsaround the flange 65 of the clutch drum 55 and at its other end isprovided with an attachment 137 for a spring 139, the other end of whichis anchored by a pin 141 to the housing 1. The spring 139 attends todraw the band 135 against the flange 65 of the drum 55 and thereby brakethe drum. In the position illustrated in FIG. 12, the brake band isprevented from engaging the flange 65 by reason of the fact that a rod143, which is also connected with the band attachment 137, draws theattachment 137, and therewith the band 135, to the left as viewed in thedrawing against the action of spring 139. The rod 143 is connected withthe safety hand grip (not shown in FIG. 12) located in the rear handleof the motor saw and takes the position illustrated in FIG. 12 when thehandle is pressed in. If the handle is released, the spring 139 willpull the band into abutment with the clutch drum 55. Attached to brakeband 135 is a strip 145 of sheet metal, the strip 145 being attached ina manner such that it lies to one side of the plane of the brake bandand slightly within the circle arc of said band. The sheet metal stripwill therefore lie in the movement plane of the lock 89. When the safetydevice is brought into function and the brake band approaches the flange65 of the clutch drum 55, the strip 145 will be engaged by the lock 89,which is thus pivoted and releases the centrifugal drive arm 77. Uponcontinued movement of the brake band 135, the band will abut the drumflange 65 and brake the same. With the embodiment illustrated in FIG.12, the drive motor is disengaged before the clutch drum, and therewiththe saw chain, are braked, whereby the motor is completely unaffected bythe operation of the safety device.

What I claim is:

l. A safety device, for a power device such as a chain saw, arrangedbetween a tool motor and a driving member such as a chain drive sprocketfor a saw chain, rotatable thereby, said driving member being freelyrotatable on the motor drive shaft and being rigidly connected with adrum cooperating with mechanical brake means; the safety devicecomprising two mechanical clutches transferring motor torque to thedriving member, namely, a centrifugal clutch and a mechanical safetyclutch, and common actuating means for the safety clutch and the brakemeans,

wherein the safety clutch comprises a drive disc fixedly attached tosaid motor drive shaft and a coupling element movably attached to thedrive disc and engageable with and disengageable from a centrifugalclutch hub yoke located adjacent said drive disc and is rotatable withrespect to the drive shaft;

said centrifugal clutch cooperating with said drum connected with thedriving member and the actuating means for the brake means cooperatingwith the drum being connected with an operating member for disengagingthe coupling element of the safety clutch.

2. A safety device according to claim 1, characterized in that the brakemeans includes a portion of the clutch drum (55) formed as a brakingdrum (65), and that a braking element (135) is drawn against the drumwhen the safety actuating means is actuated for disengaging the couplingmeans.

3. A safety device according to claim 1, characterized in that thesafety clutch is provided with a reengaging means which engages thesafety clutch when the rotational speed of the drive shaft falls tobeneath the engaging speed of the centrifugal clutch.

4. A safety device according to claim I, wherein the coupling element isactuated by a force urging the element out ofits position of engagement,and wherein the element is capable of being locked in a position ofengagement against the action of said force by means of a lock which iscapable of being released by means of the actuating means.

5. A safety device according to claim 4, wherein extending outwardlyfrom the hub yoke is a drive shoulder, and wherein the coupling elementcomprising a drive arm which is capable of being pivoted by centrifugalforce, against the action of a spring, around a pin mounted on the drivedisc, and wherein said lock is formed ofa locking arm pivotally arrangedon a second pin located on the drive disc, the locking arm whenoccupying its locking position extending substantially radially and theouter end of which is actuateable by the actuating means.

6. A safety device according to claim 5, wherein the spring actuatingthe drive arm overcomes the centrifugal force acting on the drive arm atrotational speeds below the engaging speed of the centrifugal clutch,and thereby pivots the drive arm into engagement position, and whereinthe locking arm is adapted to take its radial locking position inresponse to the centrifugal force.

7. A safety device according to claim 1, wherein the actuating meanscomprises a means for rlcasing said lock and adapted to be brought intoa lock-releasing position when a hand grip located in a handle of thetool is released.

8. A safety device according to claim 7 arranged in a motor chain saw,wherein the release means is capable of being actuated by anacceleration sensing means arranged on the motor saw and arranged torelease the lock when the saw is subjected to violent and lurchingmovements.

9. A safety device according to claim 3, wherein the brake meanscomprises a brake ring for braking said drum and being positioned aroundsaid drum, the outer surface of the ring presenting at least oneshoulder adapted to co-act with the actuating means when disengaging thesafety clutch.

10. A safety device according to claim 3, wherein the brake meansincludes a portion of said drum formed as a braking drum, and wherein abraking element is drawn against the drum when the actuating means isactuated for disengaging the safety clutch.

11. A safety device according to claim 10, in which the actuating meansincludes means mounted on the braking element for disengaging the safetyclutch.

1. A safety device, for a power device such as a chain saw, arrangedbetween a tool motor and a driving member such as a chain drive sprocketfor a saw chain, rotatable thereby, said driving member being freelyrotatable on the motor drive shaft and being rigidly connected with adrum cooperating with mechanical brake means; the safety devicecomprising two mechanical clutches transferring motor torque to thedriving member, namely, a centrifugal clutch and a mechanical safetyclutch, and common actuating means for the safety clutch and the brakemeans, wherein the safety clutch comprises a drive disc fixedly attachedto said motor drive shaft and a coupling element movably attached to thedrive disc and engageable with and disengageable from a centrifugalclutch hub yoke located adjacent said drive disc and is rotatable withrespect to the drive shaft; said centrifugal clutch cooperating withsaid drum connected with the driving member and the actuating means forthe brake means cooperating with the drum being connected with anoperating member for disengaging the coupling element of the safetyclutch.
 2. A safety device according to claim 1, characterized in thatthe brake means includes a portion of the clutch drum (55) formed as abraking drum (65), and that a braking element (135) is drawn against thedrum when the safety actuating means is actuated for disengagiNg thecoupling means.
 3. A safety device according to claim 1, characterizedin that the safety clutch is provided with a re-engaging means whichengages the safety clutch when the rotational speed of the drive shaftfalls to beneath the engaging speed of the centrifugal clutch.
 4. Asafety device according to claim 1, wherein the coupling element isactuated by a force urging the element out of its position ofengagement, and wherein the element is capable of being locked in aposition of engagement against the action of said force by means of alock which is capable of being released by means of the actuating means.5. A safety device according to claim 4, wherein extending outwardlyfrom the hub yoke is a drive shoulder, and wherein the coupling elementcomprising a drive arm which is capable of being pivoted by centrifugalforce, against the action of a spring, around a pin mounted on the drivedisc, and wherein said lock is formed of a locking arm pivotallyarranged on a second pin located on the drive disc, the locking arm whenoccupying its locking position extending substantially radially and theouter end of which is actuateable by the actuating means.
 6. A safetydevice according to claim 5, wherein the spring actuating the drive armovercomes the centrifugal force acting on the drive arm at rotationalspeeds below the engaging speed of the centrifugal clutch, and therebypivots the drive arm into engagement position, and wherein the lockingarm is adapted to take its radial locking position in response to thecentrifugal force.
 7. A safety device according to claim 1, wherein theactuating means comprises a means for releasing said lock and adapted tobe brought into a lock-releasing position when a hand grip located in ahandle of the tool is released.
 8. A safety device according to claim 7arranged in a motor chain saw, wherein the release means is capable ofbeing actuated by an acceleration sensing means arranged on the motorsaw and arranged to release the lock when the saw is subjected toviolent and lurching movements.
 9. A safety device according to claim 3,wherein the brake means comprises a brake ring for braking said drum andbeing positioned around said drum, the outer surface of the ringpresenting at least one shoulder adapted to co-act with the actuatingmeans when disengaging the safety clutch.
 10. A safety device accordingto claim 3, wherein the brake means includes a portion of said drumformed as a braking drum, and wherein a braking element is drawn againstthe drum when the actuating means is actuated for disengaging the safetyclutch.
 11. A safety device according to claim 10, in which theactuating means includes means mounted on the braking element fordisengaging the safety clutch.